Note: Descriptions are shown in the official language in which they were submitted.
w CA 02247349 1998-08-25
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ADAPTIVE ARRAY ANTENNA UNIT
TECHNICAL FIELD
The invention relates to an array antenna for use in a base station of
a mobile communication such as automobile telephone, cellular telephone
or the like and comprising an array of a plurality of antenna elements to
provide a service area defined by an angular range in a horizontal plane or
a so-called sector area, and more particularly, to an adaptive array antenna
unit having an adaptive processor which adaptively suppresses an
interference wave connected thereto.
io PRIOR ART
In the mobile communication such as automobile/cellular telephone
or the like according to the cellular system, those base stations which are
distantly spaced apart utilize identical frequencies in order to increase the
subscriber capacity so that limited frequencies can be efficiently utilized.
However, when frequencies are used repeatedly, there arises a problem of
interference noises due identical frequencies. Another issue occurs that
the subscriber capacity is degraded as the interference noises increase.
Conventional approach to suppress the interference noises has been
the use of a directional antenna for the base station antenna. An antenna
2o which exhibits the directivity in the horizontal plane is utilized, and -
techniques such as sectoring a cell or a beam tilting which varies the
directivity in the vertical plane have heretofore been employed. These
techniques achieve the effect of improving the reception SIR (signal wave /
interference wave ratio) in that the use of a directional antenna for the base
station antenna is effective to suppress interference waves from directions
other than the direction of the antenna directivity.
In addition to these techniques, an investigation is recently being
' CA 02247349 1998-08-25
made to suppress interference noises by the use of an adaptive array
antenna. An adaptive array antenna refers to the technique which
employs a plurality of antennas (an array antenna) arranged so as to be
spatially spaced apart to define adaptively a directivity having null beam
(of zero sensitivity) in the direction of an interference wave and a narrow
beam in the direction of a desired wave, thus suppressing the interference
noise level. However, in the investigation of past adaptive array antennas,
it is desired that the beam direction thus defined can be changed at will
over a broad range, and accordingly, a non-directional (or whole
1o directivity : omni-directivity) element is used for each of the antenna
elements. An arrangement in which a directional antenna is used for
individual elements which constitute together an array antenna to provide
their radiant directivity is scarcely found. Even in the CDMA system,
there has been no idea of employing an adaptive array antenna which uses
directional antenna elements.
As mentioned previously, a sectoring technique is frequently
employed in the cellular system, and a directional antenna which is adapted
to the sectored configuration is required at this end. In a conventional
system which does not employ an adaptive array antenna, an antenna of a
2o base station has a directivity in a horizontal plane, a half power width -
(hereafter referred to as beam width) of which is equal to a sector width.
Thus, an antenna having a beam width of 120° is normally used for a
120°-
sector (or 3 sector) arrangement. In an investigation which deals with the
application of a directional antenna to a prior art base station adaptive
array
antenna (see "Influences of antenna directivity in a mobile communication
base station adaptive array antenna" by Ryo Yamaguchi and Yoshio Ebine,
Academy of Communication Technical Report AP 96-131, 1997-Ol), it is
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reported that an antenna having a beam width broader than the sector angle
is required to construct sectors since the angle over which interference
waves can be rejected is narrower than the beam width of the antenna.
The investigation disclosed in this literature relates to a mobile
communication system which incorporates TDMA system as the radio
access technique, and thus reveals an outcome of investigation obtained
under a condition that there are a relatively few number of interference
waves. Currently, there is no instance of investigating a relationship
between the sector angle and the beam width under a condition that there
are an increased number of interference waves as in the CDMA system.
Thus, the use of a directional antenna has little been taken up in the
investigation of conventional adaptive array antennas, and accordingly,
there has been little disclosure on how an optimum antenna can be
constructed when an adaptive array antenna is to be used with a sector cell
for which a directional antenna is used. In particular, it is the current
status of the art that no antenna arrangement has been disclosed which can
be used in an environment that a number of interference waves are
oncoming from all directions as occurs in a system which incorporates the
CDMA as the radio access technique.
2o It is an object of the invention to overcome such problem and to
provide an optimum adaptive array antenna unit for a base station
according to the CDMA mobile communication system.
DISCLOSURE OF THE INVENTION
According to a first aspect of the invention, in an adaptive array
2a antenna unit for a base station of mobile communication in which CDMA
system is employed as the radio access technique, a service area within a
sector is defined by using antenna elements which constitute together an
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array antenna and each have a be~:m width within the horizontal plane
which is narrower than the sector angle. In particular, the service area
can be defined by a number of antenna elements greater than the number of
antenna elements (referred to as reference number) which is required when
the beam width within the horizontal plane of the antenna element is
substantially equal to the sector angle.
According to a second aspect, an antenna having a beam width
broader than the sector angle within the horizontal plane is employed as an
element. In particular, the service area can be defined by a number of
to antenna elements which is reduced from the reference number of elements.
BRIEF DESCRIPTION OF 'THE DRAWINGS
Fig. 1 is a diagram showing the directivity of an antenna which is
used in a computer simulation;
Fig. 2 shows the layout of array antenna elements or a four-element
la array antenna and a coordinate system;
Fig. 3 is a diagram illustrating a result of a computer simulation for
an error rate characteristic of a received signal as the angle of a desired
station is changed with the beam width of an array antenna used as a
parameter;
2o Fig. 4 is a diagram showing a result of a computer simulation for an
error rate characteristic of a received signal as the angle of a desired
station
is changed with the number of elements in the array antenna used as a
parameter;
Fig. 5 is a diagram showing a relationship between the element
25 beam width, the sector angle, and the number of array elements;
Fig. 6 is a schematic view showing a sector arrangement according
to a first embodiment of the invention;
CA 02247349 1998-08-25
Fig. 7 is a schematic view showing an array antenna arrangement
according to the first embodiment of the invention;
Fig. 8 illustrates the use of dipole antennas as antenna elements in
the first embodiment;
Fig. 9 illustrates the use of patch antennas as antenna elements in
the first embodiment;
Fig. 10 is a schematic view showing a sector arrangement according
to a second embodiment of the invention; and
Fig. 11 is a schematic view showing an array antenna arrangement
to according to the second embodiment of the invention.
BEST MODES OF CARRYING OUT THE INVENTION
Before describing the embodiments of the invention, a result of a
computer simulation for the directivity characteristic when a directional
antenna is applied to an adaptive array antenna base station according to
CDMA mobile communication system will be described. Specifically, an
error rate characteristic of a received signal from a mobile station as the
location of the mobile station, the directivity of each of antenna elements
which constitute an array antenna and the number of antenna elements
2o which constitute the array are changed is described, thereby indicating
that
an antenna arrangement (antenna directivity, the number of array elements)
for a desired sector angle or the present invention can be obtained.
The simulation has taken place in an environment that 36 mobile
stations (users) are laid out within a cell, each being simultaneously
engaged in communication using mutually different spread codes, so that a
condition is achieved that there are a number of interference waves.
Transmitting power from the mobile station is controlled so that a received
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power from respective mobile station is uniform among all the users. Fig.
1 shows the directivity in the horizontal plane of antenna elements used in
the simulation. The abscissa indicates the angle as normalized in terms of
beam width BW while the ordinate indicates the relative gain as normalized
by the peak power. The peak gain is chosen so that the power radiated
from the antenna remains constant if the beam width BW is changed, and the
side lobe level is chosen to be 15 dB below the peak power. A plurality of
antenna elements 11 are disposed on a line in the horizontal plane to
provide a linear array as shown in Fig. 2, with the spacing between antenna
to elements to be a half wavelength spacing, and with the principal beam
directed in a direction of 8 = 0° for all of antenna elements 11 which
constitute the array antenna and directed perpendicular to the direction of
array of the antenna elements 11 within the horizontal plane.
Fig. 3 illustrates an example of a result of calculation. This Figure
illustrates the error rate characteristic depending on the location of the
mobile station, the abscissa representing the angle of the mobile station as
viewed from the base station antenna (with the frontal direction of the array
antenna being 0°) while the ordinate represents the error rate. Because
the
transmitting power of the mobile station is controlled, the dependency on
2o the location of the mobile station does not depend on the distance between
the mobile station and the base station, thus requiring a consideration of
only the angular dependency. Respective curves shown illustrate the
characteristics when the beam width BW of the antenna element 11 is
changed in increment of 30° from 30° to 180°, all the
curves been shown
z5 for four-element array antennas. Assuming that a sector angle is
represented by an angular region in which the error rate as determined from
this Figure is equal to or less than 10-3, the sector angle will be about
40°
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when the beam width BW is equal to 30°, and in a range of beam width BW
of 60°~-180°, the sector angle is substantially equal to
90° and remains
constant, indicating a result that there is no proportionality between the
element beam width and the sector angle. An adaptive array antenna
a exhibits an excellent performance that it forms a null beam toward an
interfering station (wave) and directs its beam peak toward a desired station
(wave), but when a directional antenna element is used, the beam tracking
capability is degraded when the direction of the mobile station (or the
direction of the desired wave) shifts toward the end of the beam width.
to This is attributable to the fact that the directivity of the antenna
element 11
has its gain inherently reduced toward the beam end. It then follows that
the beam width of the antenna element can be increased in order to increase
the sector angle. However, since the interference waves are oncoming
from all directions in the CDMA system, as the beam width of the antenna
i5 element is increased, this result in receiving much more interference waves
to degrade the reception SIR, also degrading the error rate characteristic.
For these reasons, there results a consequence that the sector angle can not
be increased if the beam width of the antenna element is increased.
Fig. 4 illustrates the error rate characteristic depending on the
20 location of the mobile station in the similar manner as in Fig. 3, but in
this
instance, curves 4a, 4b and 4c show the characteristics when the number of
antenna elements which constitute the array (hereafter referred to as the
number of array elements) is chosen to be equal to 4, 6 and 8, respectively.
The beam width of the antenna element is equal to 120°. It will be
seen
2a from this Figure that as the number of array elements is increased, the
sector angle can be increased if the elements having the same beam width
are employed. When the number of elements which constitute an adaptive
CA 02247349 2001-O1-15
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array antenna is equal to N, the number of null beams which are formed in
the directions of interference waves will be equal to N-1 (this is referred to
as the freedom of the array antenna). Consequently, as the number of array
elements increases, the number of null beams formed increases, thus
improving the reception SIR and increasing the sector angle. In the present
simulation, a condition is employed that the number of interference waves
is greater than the number of array elements, and accordingly, as the
number of array elements is increased, the reception SIR is improved in a
proportional manner, which is interpreted as increasing the sector angle.
to A summary of these considerations is graphically shown in Fig. 5
where the abscissa represents the element beam width while the ordinate
represents an angle (sector angle) within which the error rate is equal to or
less than 10-3, with individual curves Sa, Sb and Sc representing
characteristics when the number of array elements is changed to 4, 6 and 8,
respectively. A rectilinear line 13 represents a line where a coincidence is
reached between element beam width and the sector angle. For example, it
will be seen that the number of array elements required when the element
beam width is 90° and the sector angle is 90° is equal to 4
while the
number of array elements when the element beam width is 120° and the
2o sector angle is 120° is substantially equal to 6. When an element
beam
width of 120° is chosen, the number of array elements required to
achieve
the sector angle of the same value 120° is substantially equal to 6,
and
when the number of array element is increased above this value, for
example, to 8, the sector angle will be substantially equal to 135° or
becomes greater than the element beam width of 120°. Conversely, when
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the number of array elements is reduced from 6 to 4, the sector angle will
be substantially equal to 85°, which is less than the element beam
width of
120°.
These illustrations indicate that (1) if the element beam width is less
than the sector angle, a service area which is broader than the beam width
can be obtained by increasing the number of array elements (as indicated in
region #1 in this Figure), and that (2) when an element beam width greater
than the sector angle is employed, the number of array elements per sector
can be reduced (as in region #2).
to In accordance with the outcome of above investigations, a first
embodiment of the invention is illustrated in Figs. 6 and 7. Fig. 6 is a
schematic view showing a sector arrangement in which a single cell is
divided into three 120°-sectors (sector #S1, #S2, #S3), with a base
station
antenna unit which incorporates an adaptive array antenna being disposed
15 in each sector. Fig. 7 shows the arrangement of a base station antenna
unit for three sectors. Antenna units BAI, BA2 and BA3 for the
respective sectors each comprise an 8-element array antenna formed by 8
antenna elements AE1 ~-AEg disposed in an array as spaced from a
reflecting plate 21. Each of the antenna elements AE1 ~-AE8 is a
2o directional antenna. The antenna element has a beam width within the
horizontal plane equal to 90° which is narrower than the sector angle.
Such beam width can be set up as desired by adjusting the spacing between
the antenna elements AEl ~-AE8 and the reflecting plate 21. The
arrangement of Fig. 7 corresponds to the region #1 shown in.
25 Fig. 8 shows the arrangement of an array antenna where half
wavelengths dipoles associated with a reflecting plate are used as antenna
elements. Each of antenna units BA1, BA2 and BA:3 for the respective
CA 02247349 1998-08-25
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sectors comprises a reflecting metal plate 21, and dipole antennas DA1~-
DA8 disposed in front of the reflecting plate 21. The distance between the
surface of the reflecting plate 21 and the dipole antennas DAl ~-DA8 is one-
quarter the wavelength ~, used, for example. In this instance, the beam
width in the horizontal plane of each antenna element is equal to about
120°. If the distance between the dipole antenna elements and the
surface
of the reflecting plane 21 is reduced, the beam width will be reduced.
Conversely, if the spacing is increased, the beam width will increase.
Fig. 9 shows the arrangement of an array antenna in which patch
to antennas (micro-strip antennas) are used as antenna elements. The
antenna comprises a dielectric substrate 22 with a metal sheet applied to its
back surface, and quadrilateral metal patch antennas PAl ~-PA$ disposed on
the front surface of the substrate as spaced from each other. When one
side of the patch antenna measures approximately one-quarter wavelength
(or more exactly ~. /4 ~ where E denotes the dielectric constant of the
dielectric substrate 22), the beam width in the horizontal plane will be
about 90°.
In addition, horn antennas may be used as antenna elements, and a
desired beam width can be obtained by choosing an opening angle of the
2o horn antenna.
In this manner, if the beam width of each of elements which
constitute together an adaptive array antenna is narrower than the sector
angle, a service area having a sector angle greater than the beam width can
be obtained by increasing the number of array elements.
Embodiment 2
Figs 10 and 11 show a second embodiment of the invention. Fig.
10 is a schematic view showing the sector arrangement where a single cell
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is divided into four 90°-sectors (sector #S1, #S2, #S3 and #S4), with a
base
station antenna unit incorporating an adaptive array antenna being disposed
in each sector. Fig. 11 shows the arrangement of a base station antenna
unit. An antenna unit for one sector is a 4-element array antenna formed
by four antenna elements AE1 ~-AE,~, with each antenna element being a
directional antenna. The antenna element has a beam width equal to 120°
which is greater than the sector angle. This arrangement corresponds to
the region #2 shown in Fig. 5.
In this manner, if the beam width of each of elements which
1o constitute an adaptive array antenna has a broader angle than the sector
angle, the number of array elements can be reduced even though the sector
angle which defines the service area will be narrower than the beam width.
Also in this embodiment, the antenna elements may be dipole antennas in
the similar manner as shown in Fig. 8 or patch antennas in the similar
~5 manner as shown in Fig. 9.
EFFECTS OF THE INVENTION
As described above, in accordance with the invention, if the beam
width of each of antenna elements which constitute an adaptive array
antenna is narrower than a sector angle, a broader service area can be
2o achieved by increasing the number of array elements. Conversely, when
antenna elements each having a beam width broader than a sector angle is
used as the element antennas, the number of array elements can be reduced
than the number of elements which would be required when using antenna
elements each having the element beam width equal to the sector angle.
25 As a consequence of these, it is possible to design an optimum antenna
arrangement for a desired sector arrangement in the base station adaptive
array antenna for CDMA mobile communication.
